Electron beam apparatus

Abstract

An electron beam apparatus comprises an electron gun having a cathode consisting of single-crystal lanthanum hexaboride and a plurality of electron lenses for projecting the cross-over image of an electron beam emitted from the electron gun as a reduced-scale image onto a sample. It further comprises a circuit for measuring the brightness and shape of the cross-over image projected onto the sample and the emission pattern of the electron beam emitted from the electron gun, and a circuit for applying a bias voltage, with which the cross-over image has a desired brightness and is round and the emission pattern is anisotropic, to the electron gun.

What we claim is: 
     
       1. An electron beam apparatus comprising: an electron gun having a Wehnelt electrode and an cathode;   heater means for heating the cathode;   optical means for projecting a cross-over image of an electron beam emitted from said electron gun onto a sample;   means for sensing a figure of the cross-over image on the sample to produce an output signal denoting the circularity of the cross-over image; and   bias means for applying a bias voltage to said Wehnelt electrode according to the output of said sensing means.   
     
     
       2. An apparatus according to claim 1, in which said sensing means further includes a brightness determining means for sensing the brightness of the cross-over image, and said bias means applies a bias voltage at which the cross-over image is circular and the brightness of the cross-over image has a predetermined value. 
     
     
       3. An apparatus according to claim 2, in which said bias means varies said bias voltage applied to said Wehnelt electrode at a constant cathode temperature for determining the bias voltage at which the brightness of the cross-over image is maximum, and if the maximum brightness at the determined bias voltage is less than a desired brightness, said heating means increases the cathode temperature to increase the brightness of the cross-over image. 
     
     
       4. An apparatus according to claim 3, in which said optical means is a cross-over reduction type having an electron lens for converting a cross-over image. 
     
     
       5. An apparatus according to claim 3, in which said optical means comprises an aperture, a first electron lens for forming a cross-over image at the aperture, and a second electron lens for projecting the cross-over image at the aperture onto the sample. 
     
     
       6. An apparatus according to claim 2, in which said bias means varies a bias voltage applied to the Wehnelt electrode at a constant cathode temperature for determining the bias voltage at which the cross-over image is round, and if the brightness of the cross-over at the determined bias voltage is less than a desired brightness, said heating means increases the cathode temperature to increase the brightness of the cross-over image. 
     
     
       7. An apparatus according to claim 6, in which said optical means is a cross-over reduction type having an electron lens for converting a cross-over image. 
     
     
       8. An apparatus according to claim 6, in which said optical means comprises an aperture, a first electron lens for forming a cross-over image at the aperture, and a second electron lens for projecting the cross-over image at the aperture onto the sample. 
     
     
       9. An apparatus according to claim 1, in which said bias means applies a bias voltage at which an anisotropic cross-over image changes to a circular cross-over image. 
     
     
       10. An electron beam apparatus comprising: an electron gun having a cathode formed of a single crystal and a Wehnelt electrode;   heater means for heating the cathode;   optical means for shaping an emission pattern of an electron beam emitted from said electron gun and projecting a shaped pattern onto a sample;   means for sensing a figure of the emission pattern on the sample to produce an output signal denoting the circularity of the emission pattern; and   bias means for applying a bias voltage to the Wehnelt electrode according to the output of said sensing means.   
     
     
       11. An apparatus according to claim 10, in which said sensing means further includes a brightness determining means for sensing the brightness of the emission pattern, and said bias means applies a bias voltage at which the emission pattern is circular and the brightness of the emission pattern image has a predetermined value. 
     
     
       12. An apparatus according to claim 11, in which said bias means varies said bias voltage applied to said Wehnelt electrode at a constant cathode temperature for determining the bias voltage at which the brightness of the emission pattern is maximum, and if the maximum brightness at the determined bias voltage is less than a desired brightness, said heating means increases the cathode temperature to increase the brightness of the emission pattern. 
     
     
       13. An apparatus according to claim 11, in which said bias means varies a bias voltage applied to the Wehnelt electrode at a constant cathode temperature for determining the bias voltage at which the emission pattern is round, and if the brightness of the emission pattern at the determined bias voltage is less than a desired brightness, said heating means increases the cathode temperature to increase the brightness of the emission pattern. 
     
     
       14. An apparatus according to claim 10, in which said optical means is a Ko/ hler illumination type including an aperture, a first electron lens for converting an image at the aperture, and a second electron lens for converting an enlarged cross-over image onto an objective lens aperture which determines the conversion angle.